1. Field of the Invention
The present invention relates generally to optical fiber communications cables specifically designed for undersea use and, more particularly, to undersea communications cables in which one or more optical fibers are disposed within a centrally located buffer tube.
2. Discussion of the Background Art
Telecommunication systems employing optical fibers as the transmission medium have become widespread because of their wide bandwidth, relatively low optical loss, and the development of optical amplifiers that do not require conversion of the optical signal into the electrical domain for amplification. Certain operating environments, however, pose specific design challenges to those seeking to efficiently utilize the unique properties and advantages of optical fibers. Systems have been specifically designed, for example, to span trans-oceanic distances to accommodate the intercontinental exchange of voice and data traffic at rates approaching 10 gigabits/s over a single optical fiber. In addition to the development of highly sophisticated techniques for transmitting and receiving the optical signals representative of this high-volume traffic, the implementation of undersea telecommunication systems has further required advances in cable design to adequately protect the fibers over a system design life typically in excess of several decades.
In designing a cable suitable for undersea use, one should have an appreciation of the external environmental and operating factors having the potential to affect the transmission carrying ability of the fiber. For example, once a cable has been deployed at the floor of an ocean, it may be subjected to high hydrostatic pressure, low temperatures, as well as the corrosive effects of sea water. One must also take into consideration the possibility of damage to the cable as, for example, might be caused by ships weighing anchor in the area of the cable or conducting commercial fishing operations. In such event, the ingress of water might potentially damage up to several kilometers of fiber in the time it would take to send out a repair vessel to splice in a new section of cable. Moreover, during the cable recovery and repair process itself, the damaged undersea cable might be subjected bending stresses, as well as tensile loads approaching 13,000 to 18,000 pounds depending on the cable type and depth of the affected section. Finally, the design of the cable must be such that it is economically practical to manufacture, with a level of quality and reliability that is repeatable.
An undersea cable design which has met with a fair degree of commercial success is disclosed by Marlier et al. in U.S. Pat. No. 5,125,062 entitled UNDERSEA TELECOMMUNICATIONS CABLE HAVING OPTICAL FIBERS and issued on Jun. 23, 1992. The cable design disclosed by Marlier et al. comprises a centrally located buffer tube within which there is disposed a sealing gel and through which extend a plurality of optical fibers. Marlier et al. indicate that the buffer tube is intended to protect the fibers from the compressive effects of undersea deployment, and that it may be either made of steel or plastic. In the event of a plastic buffer tube, Marlier et al. propose that one or more layers of strength members may be helically wound around the buffer tube so as to enhance the structural integrity of the cable. An electrically conductive tube is swaged over these strength members to carry electricity from a remote source on land to such equipment as pump lasers which disposed in optical amplifier assemblies disposed at intervals along the cable lay. A polyethylene sheath surrounds the structure.
While the Marlier et al. design is relatively easy to manufacture, no attention is given in the patent disclosure to the behavior of the fiber when the cable is placed under tensile load, nor how or whether the fiber returns to a relaxed, i.e., unstressed state after such load is removed. Accordingly, no meaningful guidance is given to the selection of material properties and other design variables so as to achieve a design which is highly robust and reliable over the broad range of operating and environmental conditions, including those to which the cable might be subjected during repair, that might be encountered over the multiple decade design life typically demanded by purchasers of undersea telecommunications systems.
The aforementioned need is addressed and an advance is made in the art, by an undersea telecommunications cable which comprises a buffer tube to protect a plurality of optical fibers disposed therein from externally applied forces. To this end, the buffer tube contains a thixotropic, water blocking gel having a viscosity and a critical yield shear stress sufficient to provide coupling, upon application of a tensile load to the cable producing up to a 0.8% cable strain, of tensile forces from the buffer tube to the optical fibers so as to induce strain in the fibers proportional to the strain induced in the tube without preventing the return of the optical fibers to a substantially unstrained condition upon the removal of such tensile load. Accordingly, should it become necessary to repair a section of the undersea cable as by retrieving it from the sea bed and inserting a spliced segment, existing sections of the fiber will be sufficiently protected from any damage which might otherwise have been caused during the cable retreival and recovery operation.
As used herein, the phrase substantially unstrained condition is intended to refer to an average residual strain in the fiber in the range of from about xe2x88x920.05 to about 0.10, the preferred value being as close to zero as possible. Since the amount of residual strain may not be uniformly distributed along the fiber, however, the condition may alternatively be defined by reference to the increase in attenuation or average loss in the fiber that is attributable to the residual strain therein. For a typical undersea telecommunication system, such average change in attenuation should be substantially close to zero as any appreciable increase, say above 0.002 to 0.005 dB/km may cause a substantial impairment to the overall system power budget.